1. Field of the Invention
The present invention relates to a photoelectric device such as a solar cell (including a photovoltaic element), a photoelectric sensor or the like. More particularly, the present invention relates to a structure and a method for mounting a photovoltaic element. The present invention also relates to a semiconductor-element mounting substrate having a circuit pattern for a semiconductor element and which is used for mounting said semiconductor element on said substrate and a method for mounting a semiconductor element on said substrate.
2. Related Background Art
In recent years, the global warming of the earth because of the so-called greenhouse effect to an increase in the content of CO2 gas in the air has been predicted.
In view of this, there is an increased demand for the development of clean energy sources with no accompaniment of CO2 gas exhaustion. As one of such clean energy sources, there can be mentioned atomic power generation. However, for the atomic power generation, there are problems that are difficult to be solved, such as radioactive wastes and the like which cause air pollution. Also in view of this, there is an increased demand for providing clean energy sources which are highly safe and do not exhaust air-polluting substances.
Under these circumstances, public attention has now focused on a solar cell in which one or more photovoltaic elements are used and which converts sunlight into electric energy as a clean energy source, because it does not exhaust contaminants and it is safe and can be readily handled.
As such solar cell, there are known have a variety of solar cells. And some of them have been using as power generation sources in practice. These solar cells include crystalline series solar cells in which a single crystal silicon material or a polycrystalline silicon material is used, amorphous series solar cells in which amorphous silicon material is used, and compound semiconductor series solar cells in which compound semiconductor material is used. Besides, there are known a variety of configurations for these solar cells to be practically used. Specifically, there are known, for instance, a frame type solar cell as disclosed in Japanese Laid-open Patent Application No. 82820/1993, a frame-less type solar cell as disclosed in Japanese Laid-open Patent Application No. 131048/1995, a roofing material-integral type solar cell as disclosed in disclosed in Japanese Laid-open Patent Application No. 177187/1996or Japanese Laid-open Patent Application No. 97727/1999, and an optical-concentration type solar cell as disclosed in Japanese Laid-open Patent Application No. 83006/1997.
For any of these solar cells, the material cost of the cell (the photoelectric conversion element) constituting the solar cell accounts for the largest rate of the cost of the solar cell. Thus, in order to reduce the cost of the solar cell, it is an important factor to diminish the use amount of the material constituting the cell (the photoelectric conversion element). The optical-concentration type solar cell is of the configuration in that in order to reduce the power generation cost by making full use of the ability of a photoelectric conversion element (a cell) used therein which is costly, sunlight is converged and condensed to several times to several hundreds times by means of a condenser lens to increase the quantity of incident light to the photoelectric conversion element, whereby diminishing the use amount of the photoelectric conversion element.
Aforementioned Japanese Laid-open Patent Application No. 83006/1997 specifically discloses a solar cell module having an optical-concentration structure in that a solar cell comprising a compound semiconductor material such as GaAs or the like is arranged on a retaining substrate constituted by glass, resin or ceramics, a reverse taper-like concaved portion whose open area being upward widened is arranged above the solar cell, and a light-converging structural body with a high refraction factor and which comprises a resin such as polystyrene and has a surface processed into a lens-like form is accommodated in said concaved portion. Separately, Japanese Laid-open Patent Application No. 231111/1995 discloses a substrate for an optical-concentration type solar cell. This substrate has a structure in that a plurality of small solar cells are connected respectively to a standard IC-type carrier comprising a dual in-line package or the like and the carriers are attached to a print substrate comprising a throughhole substrate or the like to establish electrical connection between the carriers.
FIG. 13(A) is a schematic external view illustrating an example of a conventional optical-concentration type solar power generation system comprising 20 optical-concentration type solar cell modules 115 arranged on a frame of a sun-chasing apparatus 109. FIG. 13(B) is a schematic cross-sectional view taken along the B-Bxe2x80x2 line in FIG. 13(A), illustrating the structure of one of the 20 optical-concentration type solar cell modules 115. FIG. 14 is a schematic diagram illustrating the configuration of neighborhood of given photovoltaic elements 100 in the light receiving face side of the solar power generation system shown in FIGS. 13(A) and 13(B), when viewed from above. In FIGS. 13(A), 13(B) and 14, reference numeral 100 a photovoltaic element, reference numeral 101 a circuit board, reference numeral 102 a silver paste, reference numeral 103 a heat sink, reference numeral 104 a circuit pattern, reference numeral 106 a power output electrode, reference numeral 108 a power output lead wire, reference numeral 108xe2x80x2 a power output cable, reference numeral 109 a sun-chasing apparatus and reference numeral 110 a Fresnel lens.
Particularly, the optical-concentration type solar power generation system shown in FIGS. 13(A) and 13(B) and FIG. 14 is configured in that the 20 optical-concentration type solar cell modules 115 are arranged on the frame of the sun-chasing apparatus 109 to establish a solar cell module array in which the 20 optical-concentration type solar cell modules 115 are arrayed in an arrangement comprising 5 rowsxc3x974 columns, each row comprising 4 of the solar cell modules and each column comprising 5 of the solar cell modules. The sun-chasing apparatus 109 is provided with a driving mechanism to move the frame having the solar cell module array thereon so as to chase the sun. A power generated by the solar cell module array is outputted to the outside through the power output cable 108xe2x80x2 connected to the power output lead wires 108 extending from the solar cell modules 115.
As will be understood with reference to FIG. 13(B) and FIG. 14, each optical-concentration type solar cell module 115 is provided with a photovoltaic element 100 mounted on a circuit board 101 through a pair of circuit patterns 104 provided on the circuit board 101, a Fresnel lens 110 in order to converge incident sunlight to the photovoltaic element 100, and a heat sink 103 in order to cool the photovoltaic element 100. The two circuit patterns 104 are provided respectively with an electrode 106 to which the power output lead wire 108 is connected so that a power generated by the photovoltaic element 100 is outputted to the outside. The circuit board 101 is fixed to the heat sink 103 through a silver paste 102,
Description will be made of the photovoltaic element 100 used in the optical-concentration type solar cell module 115 and the method of mounting the photovoltaic element on the circuit board 101 with reference to FIG. 15 [FIGS. 15(A) to 15(D)], FIG. 16 [FIGS. 16(A) to 16(D)] and FIG. 17 [FIGS. 17(A) to 17(C)].
FIG. 15(A) is a schematic external view illustrating an example of a single-crystal photovoltaic element used in the optical-concentration type solar cell module, when viewed from the light receiving face side. FIG. 15(B) is a schematic cross-sectional view, taken along the line C-Cxe2x80x2 in FIG. 15(A). FIG. 15(C) is a schematic cross-sectional view, taken along the line D-Dxe2x80x2 in FIG. 15(A). FIG. 15(D) is a schematic view illustrating the configuration of the back side (the non-light receiving face side) of the photovoltaic element shown in FIG. 15(A).
In FIGS. 15(A) to 15(D), reference numeral 121 indicates of a photovoltaic element comprising a single-crystal material and which is in a square form of 12 mmxc3x9712 mm and has a thickness of 125 xcexcm. The photovoltaic element 121 has a light receiving face provided with a reflection preventive film and which has a textured structure. The photovoltaic element 121 is provided with a pair of power output electrodes 122 and 123 at the non-light receiving face. Each of the power output electrodes 122 and 123 comprises a 10 xcexcm thick Al electrode deposited with an Au film having a thickness of about 0.01 xcexcm. The power output electrodes 122 and 123 are electrically connected respectively to the p-type electrode layer and the n-type electrode layer of the photovoltaic element 121.
Incidentally, for the optical-concentration type solar cell module, there is a demand that incident sunlight which is converged at a high concentration is efficiently irradiated to the photovoltaic element provided therein. In order to comply with this demand, there is known a manner that the active area of the photovoltaic element is enlarged so as to approximate 100% as much as possible by providing the power output electrodes at the non-light receiving face of the photovoltaic element. In order to make the photovoltaic element in this way, there is known a structure in that the photovoltaic element is mounted on a retaining substrate comprising a glass material, a resin material, or a ceramic material.
In the following, description will be made of an example of such structure with reference to FIG. 16 [FIGS. 16(A) to 16(D)] and FIG. 17 [FIGS. 17(A) to 17(C)].
In FIG. 16(A), reference numeral 101 indicates a circuit board which is provided with a pair of prescribed circuit patterns 104. In general, the circuit board 101 comprises a retaining substrate having a thickness of 0.5 mm to about 1 mm, and a pair of circuit patterns 104 comprising a material having a low electric resistance and which have a thickness of 0.01 mm to about 1 mm are formed. Specifically, in this case, the circuit board 101 comprises a square-shaped member made of a Al2O3 ceramic having a size of 25.4 mmxc3x9725.4 mm and a thickness of 0.64 mm The square-shaped member has opposite faces, one of which being joined with a 0.3 mm thick oxygen-free copper foil and the other face being joined with a 0.25 mm thick oxygen-free copper foil respectively by a DBC (direct bonding copper) method. And on the 0.3 mm thick oxygen-free copper foil on the face of the square-shaped member, a pair of circuit patterns 104 are formed. Reference numeral 105 indicates a joining material to join a photovoltaic element and a pair of power output electrodes extending from the photovoltaic element to the circuit board 101. As the joining material 105, a solder material or a carbon sheet is generally used. In this case, as the joining material 105, by means of a screen printing method using a metal plate, a cream solder material having a Suxe2x80x94Pb eutectic composition is applied at prescribed portions on the circuit board 101 where a pair of electrodes of the photovoltaic element which are corresponding to the foregoing pair of power output electrodes (122, 123) and the power output electrodes are arranged.
FIG. 16(B) shows an embodiment in that a photovoltaic element 100 is arranged together with a pair of power output electrodes 106 on the cream solder materials as the Joining material 105 on the circuit board 101, where the photovoltaic element 100 and the power output electrodes 106 are fixed onto the circuit board by means of the solder materials.
Now, in the case of the optical-concentration type solar cell module, as shown in FIG. 13(B), it is important that the photovoltaic element is situated at a position to oppose the Fresnel lens. For this purpose, it is required that the photovoltaic element and the circuit board, the circuit board and the housing body on which the Fresnel lens is retained, and the housing body and the Fresnel lens are arranged at proper positions in terms of the relative positional relationships.,
Separately, it is required for the power output electrodes 106 to make it possible to output a power generated by the photovoltaic element 100 to the outside with no resistance loss. Thus, a 0.7 mm thick oxygen-tree copper foil having a width of 15 mm and a length of 75 mm is used as each of the power output electrodes 106.
FIG. 16(C) shows an embodiment in that while maintaining the state in that the photovoltaic element 100 is arranged on the cream solder material as the joining material 105 of the circuit board 101 and the power output electrodes 106 are arranged on the circuit patterns 104 as shown in FIG. 16(B), the cream solder material is fused then cooled, whereby the photovoltaic element 100 and the power output electrodes 106 are fixed onto the circuit board 101. The heating condition to fuse the cream solder material as the joining material 105 differs depending on the composition or the like of the cream solder material. In the case where the cream solder material comprises a cream solder material having the Snxe2x80x94Pb eutectic composition, it is necessary to heat the cream solder material at least for 2 to 5 seconds so that the temperature of the cream solder material becomes to be more than 195xc2x0 C.
FIG. 16(D) shows an embodiment in that a further connection lead wire 108 is connected to each of the power output electrodes 106 by means of a solder or the like, for instance in order to electrically connect the optical-concentration type solar cell module involved with an adjacent optical-concentration type solar cell module. In this case, it is also required that the connection lead wire 108 is connected so that the power generated by the photovoltaic element 100 can be outputted to the outside without a resistance loss. In view of this, a copper wire having a diameter of about 3 mm and a length of about 350 mm is used as the connection lead wire 108.
FIG. 17(A) is a schematic cross-sectional view showing an embodiment in that the circuit board 101 is fixed to a heat sink 103 using a silver paste 102.
Now, in the case of the optical-concentration type solar cell module, it is required that the heat of the photovoltaic element 100 heated when converged incident sunlight at a high concentration is irradiated thereto is efficiently radiated by means of a heat radiation device such as the heat sink 103. For this purpose, it is necessary to increase the thermal conduction of the joining portion between the circuit board 101 and the heat sink 103. In view of this, it is desired to use, for example, a silver paste having a heat conductivity of more than 1 W/mxc2x7K at the joining portion. In this case, a silver paste ABLEBOND84-1 LKI-T1 (produced by Ablestik Japan Co., Ltd.) is used. And to join the circuit board 101 with the heat sink 103 by means of this silver paste is performed by applying the silver paste uniformly on the heat sink 103 by means of printing or the like at a thickness of about 0.35 mm, arranging the circuit board 101 having the photovoltaic element 100 mounted thereon on the silver paste applied on the heat sink 103 and thermosetting the silver paste for one hour by means of an oven maintained at 150xc2x0 C.
FIG. 17(B) is a schematic cross-sectional elevation view illustrating an optical-concentration type solar cell module prepared in accordance with the above-described method. FIG. 17(C) is a schematic slant view of the optical-concentration type solar cell module described in FIG. 17(B).
However, in the prior art as described in the above, there are disadvantages as will be described below.
In the method of mounting the photovoltaic element which has explained with reference to FIGS. 16(A)-16(D) and FIGS. 17(A)-17(C), there is a disadvantage in that the retaining substrate on which the photovoltaic element is mounted is costly to be similar to or more than the cost of the photovoltaic element and this makes it difficult to reduce the production cost of the optical-concentration type solar cell module. There is also a disadvantage in that the method comprises the step of mounting the photovoltaic element on the retaining substrate and the step of joining the retaining substrate having the photovoltaic element mounted thereon with the heat sink by means of the silver paste, each of these steps including the heating step, and each of these steps takes a time until the stop is completed, and therefore, this unavoidably raises the production cost of the optical-concentration type solar cell module. In addition, in the step in that the retaining substrate having the photovoltaic element mounted thereon is joined with the heat sink by means of the silver paste, in order to thermoset the silver paste, the retaining substrate having the photovoltaic element mounted thereon is introduced into and kept in the oven whose inside temperature is 150xc2x0 C., where there is a fear that the photovoltaic element suffers heat-rupture.
Further, in the case where the circuit board as the retaining substrate comprises a circuit board made of a ceramic, there is an occasion in that the ceramic layer in the ceramic circuit board is cracked due to the repetition of the heat treatment. When such crack portion is occurred in the circuit board, the heat conduction of the cracked portion is extremely decreased. This entails problems such that the heat of the photovoltaic element is not sufficiently radiated, and as a result, the temperature of the photovoltaic element is increased to cause a reduction in the power generation efficiency and the photovoltaic element is sometimes deteriorated in terms of the performance. Besides, in this case, there is considered occurrence of fusion in the solder which joins the photovoltaic element with the circuit board.
Other than the above-described disadvantages, there is also a disadvantage in that batch processing is necessitated because the components are separately supplied and therefore, it is necessary to use a prescribed fabrication apparatus for the batch processing, which is relatively costly.
Additionally, for the foregoing method of mounting the photovoltaic element, there are other disadvantages as will be described below.
That is, in the mounting method shown in FIGS. 16(A)-16(D), in the case where the photovoltaic element 100 is mounted on the circuit board 101 as the retaining substrate, even when the photovoltaic element 100 is accurately arranged at a prescribed position of the circuit board 101 by way of picture processing, there is an occasion in that when the cream solder material 105 is fused, part of the cream solder material and part of the flux contained in the cream solder material are issued to displace the position of the photovoltaic element 100.
Now, for a photovoltaic element used in the optical-concentration type solar cell module, the thickness thereof is required to be very thin to be, for instance, 150 xcexcm or less for the reason that sunlight which is impinged from the light receiving face of the photovoltaic element is necessary to irradiate to the p-type electrode layer and the n-type electrode layer of the photovoltaic element which are situated in the vicinity of the power output electrodes provided at the non-light receiving face of the photovoltaic element.
In the case where such photovoltaic element is mounted on the circuit board as shown in FIGS. 16(A)-16(D), when the cream solder material 105 is fused, here is sometimes an occasion in that part of the cream solder material or/and part of the flux contained in the cream solder material are issued to deposit on the light receiving face of the photovoltaic element.
Separately, in order to perform accurate positioning for the components to be mounted on the circuit board as the retaining substrate and in order to prevent the fused cream solder material from being issued, there is known a method in that a resist comprising an epoxy resin or the like is formed on the circuit board. However, in the optical-concentration type solar cell module, incident sunlight is converged and condensed to be several times to several hundreds times by means of a condenser lens and because of this, there is an occasion in that the temperature of a portion of the circuit board (having the photovoltaic element mounted thereon) which receives such condensed sunlight is extremely increased to reach several hundreds centigrade (xc2x0 C.). In this case, a problem is liable to occur in that the resist formed on the circuit board suffers from extreme heat deterioration or it is thermally decomposed to disappear. In addition, when the resist is thermally decomposed, foreign matter is generated to deposit on the light receiving face of the photovoltaic element.
Therefore, the method in that the resist is formed on the circuit board is not always effective.
The present invention has been accomplished as a result of extensive studies by the present inventor in order to solve the foregoing problems in the prior art.
An object of the present invention is to provide a mounting structure in which a photovoltaic element is mounted together with a metal body for outputting a power generated by said photovoltaic element to the outside, said photovoltaic element having a light receiving face and a non-light receiving face and having a pair of electrodes on said non-light receiving face, said metal body having a first surface and a second surface opposite said first surface, wherein said photovoltaic element is joined to said first surface of said metal body and an electrically insulative joining member is joined to said second surface of said metal body.
In this mounting structure, the metal body is capable of being a heat spreader which has a heat radiation function. And the metal body makes it possible to readily output a large electric current.
Another object of the present invention is to provide a method for mounting, a photovoltaic element, a metal body for outputting a power generated by said photovoltaic element to the outside, and a heat radiator for radiating heat generated in said photovoltaic element due to receipt of light irradiation by said photovoltaic element, said photovoltaic element having a light receiving face and a non-light receiving face and having a pair of electrodes on said non-light receiving face, said metal body having a first surface and a second surface opposite said first surface, said method including at least an element-joining step (a) of joining said photovoltaic element to a prescribed position on said first surface of said metal body by means of a joining material and a heat radiator-joining step (b) of joining said heat radiator to a prescribed position on said second surface of said metal body by means of an electrically insulative joining material.
This mounting method enables to realize aforesaid mounting structure in which the photovoltaic element is mounted.
A further object of the present invention is to provide a semiconductor element-mounting substrate for mounting a semiconductor element thereon, said mounting substrate comprising a retaining substrate having a circuit pattern for said semiconductor element, said circuit pattern having an electrode-joining portion for joining electrodes said semiconductor element and an external terminal-fixing portion for fixing an external terminal wherein said electrode-joining portion is electrically joined to said external terminal-fixing portion and said electrode-joining portion and said external terminal-fixing portion are respectively electrically insulated, said electrode-joining portion being formed to be greater than an electrode portion of said semiconductor element, and a groove being provided between said electrode-joining portion and said external terminal-fixing portion. Particularly, said mounting substrate comprising said retaining substrate and a lead frame joined to said retaining substrate, said lead frame having a circuit pattern for said semiconductor element, said circuit pattern having said electrode-joining portion where a pair of power output electrodes of said semiconductor element are joined and said external terminal-fixing portion which is electrically joined to said electrode-joining portion, said electrode-joining portion being formed to be greater than said electrode portion of said semiconductor element, and a groove being provided between said electrode-joining portion and said external terminal-fixing portion.
The semiconductor element-mounting substrate thus structured is greatly advantageous in that even when the semiconductor element mounted is a thin type photovoltaic element which is used under condition with irradiation of a highly condensed light, the mounting substrate makes it possible to desirably mount said photovoltaic element thereon without displacing the originally arranged position of said photovoltaic element and while preventing the cream solder material or/and the flux contained the rein from depositing on the light receiving face of photovoltaic element.
A still further object of the present invention is to provide a method for mounting a semiconductor element on a retaining substrate having a lead frame joined thereto, said method comprising: a step of forming a circuit pattern for said semiconductor element at said lead frame, said circuit pattern having an electrode-joining portion where a pair of power output electrodes of said semiconductor element are joined and an external terminal-fixing portion for a fixing an external terminal, said electrode-joining portion being electrically joined to said external terminal-fixing portion, said electrode-joining portion being capable of becoming to be in a form which is greater than an electrode portion of said semiconductor element; a step of forming a grove between said electrode-joining portion and said external terminal-fixing port on; a step of arranging a cream solder material at a prescribed position on said lead frame; a step of arranging said semiconductor element at a prescribed position on said lead frame where said cream solder material is arranged; a step of fusing said cream solder material to connect the electrodes of said semiconductor element to said lead frame; and a step of joining the lead frame having the semiconductor element mounted thereon with the retaining substrate.